Two tribes in Northeast India are as demographically and genetically similar as they could get, with one big difference: One is patrilineal and the other matrilineal. Guess which one had a bigger gender gap in spatial reasoning tests?

According to Wired, a study comparing spatial reasoning performance in both tribes found that "in the patrilineal society, men outperformed women on this task, taking 35 percent less time to complete it than the females. That difference vanishes when the test is given to the matrilineal tribe members; there was simply no significant difference between the sexes."

Interestingly, a previous study found that countries with lower gender equality composite scores also had a smaller gender gap. Boys performed far better than girls on math tests in Turkey and Korea, which both scored low for overall gender equality, but in Iceland, girls outscored boys by 2 percent. Norway and Sweden had very small gaps.

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The more recent study "takes advantage of a convenient natural experiment," according to Wired: the existence of two tribes with the same "agrarian lifestyle and diet," who also "reside in close proximity, and DNA tests indicate that they are closely related." But here's the difference:

The Karbi are patrilineal; women do not typically own land, and the oldest son inherits the family's property after the death of the parents. The Khasi could not be more different in this regard. Men are not allowed to own land at all, any money or goods earned by a male are handed over to his wife or sister, and inheritances go to the youngest daughter in the family.

(Side note: Can someone rewrite any Jane Austen novel to be set among the Khasi?) A total of 1300 villagers took spatial reasoning tests. In explaining the lack of gender gap in matrilineal society as opposed to men outpacing women in the patrilineal society, the authors found that the level of education "accounted for a third of the difference," and property ownership also made a difference. Other factors, such as inheritance, had no correlation.

"There may yet be places where innate differences in math and spatial reasoning exist between the sexes, but those places are becoming ever smaller," concludes Wired. But here's another question: Why is it that the matrilineal society didn't show women outpacing men, but rather relative equality? And could we introduce J.C. Penney t-shirts as an experimental variable?